Germany international Reus has missed most of the year already after suffering a groin problem in February, yet his comeback has now been delayed due to another injury.
"Borussia Dortmund will still have to be without Marco Reus," the club said in a statement.
"The captain has problems with tendon inflammation at the same injured groin muscle. He will be out for an indefinite period.
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"Reus has worked intensively in recent months and has taken steps forward. But he still has problems with some movements."
Dortmund finished second behind Bayern Munich, who won a record eighth successive league title. The new Bundesliga season starts on September 18.
The Ruhr valley club last won the league in 2012 under then coach Jurgen Klopp before the arrival of Reus.
The 31-year-old has been plagued by several major injuries and missed Germany's winning 2014 World Cup run.